The invention pertains to the arrangement of a fuze in a solid electrolytic capacitor such as a tantalum capacitor and to an improvement to the process and to the capacitor described in my parent U.S. Pat. No. 4,899,258 issued Feb. 6, 1990.
In order to reduce the detrimental effects of a short circuit in a circuit containing one or more solid electrolytic capacitors, a fuze is integrated within these capacitors themselves while limiting any resulting dimensional increase.
It is first recalled that a solid electrolytic capacitor, in particular the type made with tantalum, mainly comprises a porous pellet (anode) from which an anode wire projects, and which is completely or partially covered with various layers in practice there are layers of dielectric/oxide, of manganese dioxide layers essentially filling the pores of the anode and forming the solid electrolyte, and a conductive layer forming the counterelectrode (cathode). This structure forms a capacitor body which is coated in an electrically insulating material after attaching lugs to the electrodes that are terminated by output terminals. This process produces very compact capacitors with a simple geometric form (cylinder or rectangular parallelepiped).
Various types of built-in fuzes for capacitors of this type are already known. For example, U.S. Pat. No. 4,107,762 describes a capacitor in which the lug forming the negative terminal is connected to the counter-electrode by a fuze embedded in the insulating coating. This fuze is made of a double strip of aluminum and ruthenium-palladium. In the event of a short circuit, the heating of this fuze exothermally produces an alloy of these elements and an electrical disconnection between this negative terminal and the counter-electrode. This solution, which gives satisfaction in numerous cases, has the drawback of a large variation in the effective or useful lengths of these fuzes, and therefore in the associated current thresholds.
To correct this drawback, it was later proposed, in U.S. Pat. No. 4,224,656 to use a fuze block comprising an insulating block with parallel faces through which a hole is made and a fuze wire arranged parallel to the axis of this hole, the effective length of which is therefore determined very accurately. The precision of this fuze block is improved by the fact that this effective fuze wire length is placed in a thermally insulating air pocket which prevents the heat generated in this wire due to the Joule effect from diffusing into the capacitor and delaying the disconnection of the circuit The manufacturing of such blocks is however long and delicate.
For a greater manufacturing ease and comparable performance level, the document EPO No. 232 868 proposes a fuze element comprising an insulating plate to which two geometrically very accurate disjointed conductive layers are connected by a fuze wire placed in a given location, allowing its effective length to be accurately determined. This fuze wire is coated in a thermally insulating elastic substance, such as silicon resin, in which bubbles are formed, thereby improving the thermal insulation.
It may be noted that the documents U.S. Pat. No. 4,224,656 and EPO No. 232 868 describe a calibration of the effective length of the fuze wire, using an apparently necessary additional support element during the manufacturing, so as to provide a predetermined spacing of the zones where the ends of the fuze wire should be attached.
My parent patent U.S. Pat. No. 4,899,258 mentioned above simplifies even more the integration of a fuze in a solid electrolytic capacitor, and therefore reduces the cost for a comparable performance level, by eliminating any intermediary support element inserted between the negative connecting lug and the counter-electrode. It proposed the arrangement of a predetermined effective length of a calibrated fuze wire between two electrically insulated sections of this lug, which is directly connected by one of its sections to the counter-electrode.